Method for selectively implementing low threshold voltage transistors in digital logic designs

ABSTRACT

A system and method for selectively replacing standard threshold voltage devices with low threshold voltage devices in a digital logic design. The system identifies at least one path having a first timing value, the path having a plurality of standard threshold devices. The path is reverse traversed, or otherwise analyzed or traversed, to identify at least one of the standard threshold devices to possibly replace with a corresponding low threshold device. The system also determines a timing value for the path associated with replacing the at least one standard threshold device with the corresponding low threshold device. Depending the analysis, the standard threshold device may be replaced with a low threshold device, such as when the path timing improves by replacement. Such replacement may be used in various paths, such as paths considered critical paths in a digital logic design.

FIELD OF THE INVENTION

Aspects of the invention relate generally to the field of digital logicdesign and more particularly to the field of high speed, low powerdigital microprocessor design.

BACKGROUND

In digital logic design, engineers often use pre-defined standard cellspulled from an electronic library (or database) in order to avoidconstantly redesigning often used elements. These standard cells containlayouts for many logic functions and logic gates that have beencarefully designed and optimized for various applications. Commonly usedcells have multiple variations, including, for example, a standard cellthat has been modified for maximum speed (most likely at the expense ofsome other circuit characteristic, such as power consumption), forminimum power consumption (again at the expense of some othercharacteristic), and so forth. These modified standard cells allowcircuit designers more options and flexibility in creating digitalcircuits while still avoiding the costly procedure of redesigning cells.

Ideally, a digital logic design runs at a very high speed but consumesvery little power (thereby producing very little heat). Usually,however, the faster a particular element in a design runs, the morepower it consumes. Engineers must thus decide whether they want tosacrifice speed for power, or power for speed. One method commonlyemployed to increase propagation speeds of individual devices logicelements on a critical path (a path that needs to be optimized forspeed) is to lower the threshold voltage (or, in other words, theturn-on voltage) of devices within those logic elements on the criticalpath (the path that needs to be optimized for speed) in a digitaldesign. While this technique increases the switching speed of theindividual devices in the logic element (thereby reducing the delayintroduced into the critical path by those devices), the low thresholddevice consumes much more power due to the devices turning on earlier aswell as the current leakage when the devices are off.

One prior art solution is to simply implement a standard cell librarywith non-mixed threshold voltage cells and mixed threshold voltage cells(i.e. at the transistor level) having footprints no larger than thenon-mixed cells, wherein an implant overlay of oxide thickness and spacebetween first and second threshold voltage devices match a minimumcontacted space. This solution, however, fails to account for a methodof intelligently selecting when to use non-mixed and when to use mixedcells in a design.

SUMMARY

One aspect of the present invention involves a method for selectivelyreplacing standard threshold voltage devices with low threshold voltagedevices in a digital logic design. The method involves identifying atleast one path in a digital logic design, the at least one path having aplurality of standard threshold devices, the at least one path having afirst timing value. The method further involves reverse traversing thepath to identify at least one of the plurality of standard thresholddevices to replace with a corresponding low threshold device. The methodproceeds to determine at least one second timing value for the pathassociated with replacing the at least one standard threshold devicewith the corresponding low threshold device. Finally, depending on theoutcome of the analysis, the method involves replacing the at least onestandard threshold device with the corresponding low threshold device,such as when the second timing value is less than the first timingvalue.

Aspects of the invention may further involve a system for selectivelyreplacing standard threshold voltage devices with low threshold voltagedevices in a digital logic design. The system may involve a timingengine and other components, alone or collectively configured to obtaina first timing value for at least one path in a digital logic design,the at least one path having a plurality of standard threshold devices.The timing engine and other components further configured to reversetraverse the path to identify at least one of the plurality of standardthreshold devices to replace with a corresponding low threshold deviceprovided in a database of low threshold devices. And, the timing enginefurther configured to determine at least one second timing value for thepath associated with replacing the at least one standard thresholddevice with the corresponding low threshold device in order to replacethe at least one element with the corresponding element when the secondtiming value is less than the first timing value, or based on some othercriteria.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a circuit diagram of a sample digital logic circuitwith a critical path.

FIG. 2A illustrates a second circuit diagram of an NAND3 cell.

FIG. 2B shows a symbol often used to represent the NAND3 cell of FIG.2A.

FIG. 2C is a table indicating the speed improvements that can beachieved by using one or more low voltage devices in place of standardvoltage devices in the NAND3 cell of FIG. 2A.

FIGS. 3A-3C illustrate a third, fourth, and fifth circuit diagramsillustrating three different implementations of a NAND3 cell with someor all devices changed to LVT transistors.

FIG. 4 is a block diagram of a timing analysis engine.

FIG. 5 illustrates a flowchart of a method for selectively implementingLVT devices based on data obtained from the timing engine.

FIG. 5 a is a portion of a table showing possible timing improvements ifLVT is implemented in certain cells in a digital design.

FIG. 6 illustrates a flowchart of a method for creating a table ofpossible timing improvements based on data obtained from the timingengine.

FIG. 7A illustrates a sixth circuit diagram illustrating a NAND3 cellthat has only been modified by replacing some SVT transistors with LVTtransistors.

FIG. 7B illustrates the NAND3 cell of FIG. 7A wherein the cell has beenfurther modified to improve performance.

DETAILED DESCRIPTION

Aspects of this disclosure provide systems and methods for increasingprocessor speed and overall performance at a reduced power consumptioncost by selectively implementing Low Threshold Voltage (Low VT, or LVT)devices in place of Standard Threshold Voltage (Standard VT or SVT)devices in digital logic circuits. As described below, some aspects ofthis disclosure describe implementing selective SVT to LVT conversionsat the device (transistor) level, as opposed to at the logic cell level,in order to more effectively increase processor speed and overallperformance. Other aspects of this disclosure describe methods foroptimizing mixed and non-mixed threshold logic cells through redesign(where mixed threshold cells may be cells with any variety of SVT or LVTdevices, and non-mixed threshold cells have only SVT or LVT devices).

Digital logic designers constantly face the challenge of balancingdemands to increase processor speed and overall performance withcompeting demands to reduce power consumption and heat. In order to meetspecified requirements, they often tweak their designs throughout thedesign process. Some of these adjustments occur following a timinganalysis of the design. The timing analysis results indicate, amongother things, paths in a design that do not meet timing specificationsand that need to be changed. As described below, so called “criticalpaths” are paths along which the signal propagation time exceeds themaximum propagation delay and that thus need to be optimized to reducethe delay in order to ensure timely arrival of signals. Aspects of thisdisclosure provide systems and methods for decreasing the propagationtime of devices on critical paths, as well as other paths, byselectively implementing LVT devices in place of SVT devices, orselectively not implementing LVT devices in place of SVT devices whenthe corresponding LVT device is slower than its SVT counterpart.

Digital logic circuits typically include many small electronic devices,such as transistors. The transistors may be Field Effect Transistors(FET) or more specifically Metal Oxide Semiconductor Field EffectTransistors (MOSFET). These transistors are often in a complimentaryarrangement known as CMOS (Complimentary Metal Oxide Semiconductor).CMOS transistors may be grouped together in various hierarchicalstructures. For example, several transistors may be arranged to computethe logic function of any number of inputs, and provide the result as anoutput. Such arrangements of transistors are often referred to as logicgates, elements, cells (logic level), and so forth. Logic gates or cellsmay further be grouped into more complex blocks. The complex blocks maybe further arranged on a monolithic Integrated Circuit (IC), and severalICs may further be combined in order to provide some specificfunctionality, such as in a cell phone.

Digital logic designs may involve both clocked elements (such as flipflops, etc.) and non-clocked elements (such as NAND and NOR logic gates,inverters, etc.), and may also include discrete elements (such ascapacitors, resistors, etc.). Often times in sequential logic design,several non-clocked logic elements will be placed between two or moreclocked elements. Clocked elements usually require input signals to bestable a certain amount of time before the clock edge arrives (known asthe setup time), and require them to be stable for a certain amount oftime after the clock edge arrives (known as the hold time). Setup andhold times vary, but may be from a few nanoseconds to several hundredpicoseconds for some modern flip flops. As an electronic signal leaves afirst clocked element and proceeds through several non-clocked elements,propagation delays are introduced by each non-clocked element. If alarge amount of delay is introduced as the signal propagates, the signalmay not reach the second clocked element in a timely manner (i.e. it mayviolate the setup or hold time of a subsequent clocked element byarriving too close to the clock edge). This may cause the clockedelement to enter a metastable state, or may result in a corruptedsignal. On the other hand, if the signal arrives early enough at theflip flop in order to satisfy its setup time, and remains stable for atleast the hold time (i.e. the signal arrives in a timely manner), theremay be no metastability problem due to timing violations.

From the discussion above, it can be seen that the amount of time asignal takes to propagate down any given path depends on both the numberand type of elements it passes through. Furthermore, the operatingfrequency of a design will limit the maximum propagation delay.Specifically, the sum of the maximum propagation delay and delaysassociated with the surrounding clocked elements (setup time, hold time,and data propagation through the clocked element) can be no greater thanthe inverse of the operating frequency. Hence, the term “critical path”as introduced above may thus refer to any path in a design that exceedsthe maximum propagation delay.

FIG. 1 shows an example of a critical path 110 (shown as a bolded linebetween logic elements) in a sample digital logic design 100. Thecritical path begins at the output of a clocked element 120, such as aflip-flop, and continues through various logic gates 142-154 until itarrives at another clocked element 130. If a large delay is introducedby elements 142-154, the signal propagating along the critical path 110may not reach clocked element 130 in a timely manner as described above.

One way to reduce propagation delays on any given that can be donefairly late in the design process, is to increase the speed of some orall of the devices in the logic cells on that path by replacing SVTtransistors with LVT transistors. LVT devices increase switching speedsof digital logic circuits by lowering the threshold voltage or thevoltage required to turn a transistor ‘on’ (and ‘off’). The delayintroduced by an LVT element may be, in some cases, 20-30% shorter thanthe delay introduced by a corresponding SVT element. However, this delayreduction is often accompanied by side effects, such as an increase inpower consumption and output conflict creation, as described below inmore detail.

The delay associated with critical path 110 in FIG. 1 may be reduced byusing an LVT NAND3 cell in place of the SVT NAND3 cell 154. Note that aCMOS implementation of a NAND3 gate is a combination of MOSFETs arrangedto compute and output the inverse of the logical AND function of threeinputs. Specifically, 3 p-type transistors are arranged in parallel andcoupled to 3 n-type transistors in series. Also, the NAND3 gate is onlyone of a vast array of logic elements that may be implemented in adigital circuit.

Generally speaking, the threshold voltage is the voltage at which atransistor switches between on (conducting current) and off (notconducting current), and is part of the characteristics of a transistoras defined by the process technology. The threshold voltage can beadjusted in several ways, including by adjusting doping levels, dopants,oxide thickness, oxide material, and so forth. The threshold voltage ofa transistor is usually considered to be the gate voltage at which aninversion layer forms at the oxide and substrate interface and thetransistor conducts current sufficient to be considered ‘on’. Thus, whenthe applied gate voltage is below the threshold voltage, the transistoris ‘off’, although some current may flow from drain to source (leakagecurrent). When the applied gate voltage equals or exceeds the thresholdvoltage, an inversion layer is formed which conducts current betweensource and drain, and the transistor is ‘on.’

Note that many devices, or even entire cells, are generally notconsidered appropriate candidates for LVT replacement because using LVTdevices in place of SVT devices would likely change functionality. Suchcells may include, for example, transmission gates, latches, and soforth. Other elements generally not appropriate for SVT to LVTconversion include logic elements on paths which are sensitive to noise.Generally speaking, because transistors do not turn on until thethreshold voltage is reached, the threshold voltage allows for a certainamount of noise (up to the threshold voltage) on a particular nodewithout altering performance. If a path or node is prone to receivenoise that may alter performance, or if subsequent logic cells areunable to filter out glitches arising from a preceding block's increasedpropensity to glitch because of noise, LVT replacement may not beappropriate. Therefore, some aspects of this disclosure provide for onlyreplacing SVT devices with LVT devices if the replacement will not causenoise problems or change functionality.

In order to take advantage of the speed increase of LVT devices whileminimizing the negative side effects associated therewith, some aspectsof this disclosure provide systems and methods that selectivelyimplement LVT devices in some cells (i.e. by using LVT devices in onlypart of each cell while maintaining SVT devices in the other part). Inaddition, because the timing critical path is often only critical oneither the rising or falling edge of the clock, some aspects of thisdisclosure only implement LVT devices for the transistors thatcorrespond to the rising or falling signals (replacing p-type SVTtransistors with their LVT counterpart improves rising time, whilereplacing n-type SVT transistors with their LVT counterpart improvesfalling time). Thus, aspects of the disclosure move the decision to doSVT to LVT conversions down to the device (transistor) level, in orderto more effectively use LVT devices, thereby improving speed butavoiding some of the problems associated with LVT use described herein.

FIGS. 2A-2C illustrate some of the effects of selectively replacing SVTdevices with LVT devices at the device (instead of cell) level. Morespecifically, FIG. 2A shows a circuit diagram 200 of a NAND3 cell, andFIG. 2B is the logic symbol 240 for the NAND3 gate 200. FIG. 2C is atable indicating the rising or falling edge speed improvements fromselectively replacing SVT devices with LVT devices in the NAND3 cell ofFIG. 2A. The NAND3 FET gate 200 is one example of the NAND3 gate 154 ofthe circuit path in FIG. 1. For each SVT transistor replaced by an LVTtransistor, there is a corresponding improvement in either the rising orfalling edge of the output signal for based on each individual inputsignal. Replacing p-type transistor 210 with an LVT p-type transistorimproves the rising time of signal A. Replacing p-type transistor 212with an LVT p-type transistor improves the rising time of signal B.Replacing p-type transistor 214 with an LVT p-type transistor improvesthe rising time of signal C. Replacing n-type transistor 216 with an LVTn-type transistor improves the falling time of signal A. Replacingn-type transistor 218 with an LVT n-type transistor improves the fallingtime of signal B. Replacing n-type transistor 220 with an LVT n-typetransistor improves the falling time of signal C.

FIGS. 3A-3C show three different implementations of a NAND3 cell, eachimplementation with a different configuration of SVT and LVTtransistors. For purposes of illustration, a NAND3 FET cell is againused. FIG. 3A shows a NAND3 cell 310 where all of the transistors havebeen replaced with LVT transistors. FIG. 3B shows a NAND3 cell 320 whereonly the transistors associated with input B have been changed to LVT.FIG. 3C shows a NAND3 cell 330 where only the p-type transistorassociated with (the rising edge of) signal B has been changed to an LVTtransistor. Each of these cells may be referenced in a standard celllibrary with their corresponding characterizations, such as the improvedrising and falling time for input B associated with the NAND3 cellimplementation in FIG. 3B.

In order to intelligently select standard devices to replace with LVTdevices, a timing analysis of a design with no LVT and a timing analysisof a design with full LVT is typically performed. The informationgenerated during the timing analysis may subsequently be used todetermine where LVT devices should be implemented in place of SVTdevices. FIG. 4 is an illustration of a timing engine 400 that embodiesone aspect of a timing analysis that generates such information. Timingengine 400 may run a simulation of the digital logic design in order toproduce timing information. Timing engine 400 may take as inputsnetlists 405 of circuit designs, constraints 410, timing models 420(including SVT cell timing models 422 and LVT cell timing models 424),and user commands 430.

Netlists 405 are structural representations of digital logic circuitdesign. They typically describe instances of devices and elements(including transistors, capacitors, inductors, resistors, and so forth),the nets or wires that connect the devices and elements, and may alsoinclude some attributes of the devices and elements. A constraints file410 may include, for example, design rules, timing exceptions, parasiticinformation, operating condition extremes, and so forth, all of whichspecify limitations on the design during simulation. Timing models 420contain behavioral information for each different type of device orelement in a digital logic design. The timing models are obtained bycharacterizing devices or elements to obtain delays, slew, skew,capacitance, setup and hold times, and other behavioral informationabout the device or element needed to simulate the design and obtain atiming analysis. Timing engine 400 may include timing models 420 forboth SVT elements 422 and LVT elements 424.

The user commands 430 may instruct the timing analysis engine 400 to runtiming analysis on the netlists 405 using either SVT cell timing models422 or LVT cell timing models 424. If timing analysis is done for bothSVT and LVT cells, the timing engine 400 outputs timing analysisinformation which may include, among other things, the original slackassociated with each element in each path of the circuit with SVTimplemented 535, the slack associated with each element in each path ofthe circuit with LVT implemented 440, and the propagation delay for theworst path 445. This timing analysis information may be used todetermine which SVT cells to replace with either full or partial LVTcells within the circuit as shown in FIG. 5.

FIG. 5 shows a flowchart of a method 500 of using the data from timingengine 400 to selectively implement partial or full LVT devices inreplacement of conventional SVT devices in a digital logic design.First, each path between clocked circuit elements in a design isassigned a path number 502. Providing each path with a path number 502provides a convenient way to step through all of the different elementsand paths in a digital logic design. Assigning a path number 502 isoften automatically done by timing engine 400.

Next, a timing analysis of the original design 504 (i.e. with no LVTdevices) is performed using timing engine 400. The information obtainedfrom the timing analysis 504 is the rising and falling propagation delayfor each pin pair of all devices on all paths of the original digitallogic design. A full chip path report may also be obtained from thetiming analysis, which includes propagation times for every path in thedesign. In some instances, the full chip path report may be sorted bythe required cycle time, and the path numbers reassigned in descendingorder beginning with the path having the longest propagation dely.

Every SVT device is then replaced with its corresponding LVT device, ifit exists, and a timing analysis of the design with full LVT isperformed 506. This second timing analysis 506 provides the rising andfalling propagation delays for each pin pair of all devices on all pathsof the design. An LVT swap improvement table is then generated 508, bysubtracting the propagation delay of the full chip LVT timing analysisfrom the propagation delay of the original design for every device onevery path of the design. The LVT swap improvement table is in essence atable of all of the possible timing improvements in a design.

FIG. 5 a shows an example of an LVT swap improvement table 550. Thetable includes a column for the slack time, which edge the improvementin delay is for, the instance name of the cell, the original cell type,the LVT cell name, the propagation delay improvement, and the pathnumber, where each row has data for a given cell along a path. The LVTswap improvement table in FIG. 5A is only a small sample with many edgesof many cells excluded. A comprehensive LVT swap improvement table canbe quite large (depending on the design, it can be several gigabytes),and thus only a sampling is shown in FIG. 5A.

Each path is then reverse traversed in order to update the cycle timefor all of the paths in the digital logic design in step 504, asdescribed in more detail below in FIG. 6. The cycle time of a givenpath, in one implementation, includes the propagation delay of the path,the setup and hold time of the destination flop (e.g., flop 130) and theclock skew between the source flop (e.g., flop 120) and the destinationflop (e.g., flop 130) During the reverse traversal, the path reportgenerated by timing analysis 504 is updated based on the effects ofreplacing each individual SVT device to an LVT device.

Finally, the updated path report is sorted in order to determine theworst cycle time (i.e., includes the longest propagation delay) as wellas what will be the most beneficial LVT swaps 512 (i.e., reduces thepropagation delay). For example, the table may be sorted according tothe amount of time improvement gained by using LVT instead of SVT in aparticular cell on a particular path. A designer using such a table maywish to implement LVT in all of the devices where there is at least acertain time gain obtained by using LVT. For example, the designer maychose to implement LVT devices whenever there is a time improvement of 6picoseconds or more. In this way, LVT can be selectively implemented 514to improve speed with little increased power consumption (because feweroverall SVT devices are replaced with LVT devices). In other words, LVTis selectively implemented in such a way so that timing is improved at asmall expense (in terms of the negative side effects that oftenaccompany LVT use) is incurred.

Combining the updated path report with design specifications (such as aspecified operating frequency) may reveal that even if LVT isimplemented everywhere appropriate, the specified operating frequencycould not be reached. For example, this may be the case where a criticalpath with conventional SVT implemented has a propagation delay of 10nanoseconds, the critical path with all LVT implemented has a delay of 7nanoseconds, but the cycle period for the specified operating frequencyis only 6 nanoseconds. Here, because even full LVT implementation forall devices on the path cannot bring the propagation delay down belowthe period, other redesign techniques are needed (such as inserting aflip flop or latch somewhere in the path). The information generated bymethod 500 can be used in analyzing a great deal of information aboutthe timing of the circuit, including determining possible timingimprovements.

Further, once the updated path report has been created in step 512,selectively implementing LVT in step 514 will depend on a number offactors, such as the availability of partial LVT cells in the standardcell library being used. As described below, a standard cell library maycontain cells with only SVT devices, cells with only LVT devices, andcells that have some LVT and some SVT devices. There may be severaldifferent implementations of the partial LVT cells for each cell type,such as all of the n type transistors being replaced with LVT devices,all of the p type transistors being replaced with LVT transistors, onlythe pins associated with one or more of the inputs being replaced withLVT transistors, and so on. The circuit diagram in FIG. 3B, for example,illustrates the circuit diagram for a NAND3 cell where only the pinsassociated with input B are LVT devices. FIG. 3C illustrates the circuitdiagram for a NAND3 cell where only the p type transistor associatedwith input B is an LVT device. These different implementations areexamples of the many different combinations of LVT and SVTimplementations that may be included in a standard cell library, eachimplementation having a different characterization, such as improvedrising time, improved falling time, etc. Thus, the selectiveimplementation depends greatly on the types and varieties of cellsavailable. Cells where fewer SVT devices have been replaced by LVTdevices, as opposed to cells where more or all of the devices have beenswitched to LVT devices, will more efficiently reduce the delay, unlessthere are several critical paths going through the same device (in whichcase the cell may be replaced with a full LVT cell). In one embodiment,step 514 involves selectively implementing LVT devices in a design basedon the updated path report and using the closest match of the desiredLVT cell available in the library.

FIG. 6 illustrates a flowchart of a method of reverse traversing pathsusing timing analysis information to update the path report aspreviously described in step 510. This method begins by setting avariable N to the total number of paths in the digital circuit 602.Using the digital circuit in FIG. 1 as an example, N would be set to 1because the circuit has only 1 path. FIG. 1 is referenced to provide anexample of processing a circuit path according to the method of FIG. 6;however, the method is applicable to other paths.

Path N is then reverse traversed 604 as will be explained in thefollowing steps (reverse traversal is generally a process of steppingbackwards through a design). Another variable M is set to the number ofcells on path N (which may be the number of non-clocked elements betweentwo clocked elements) 606. For the digital circuit in FIG. 1, M would beset to 7, because the circuit has seven cells (142, 144, 146, 148, 150,152, and 154) between clocked elements 120 and 130. Here, cell “M”refers to the cell “M” positions after the first clocked element (so theOR2 cell 142 is cell “1”, the NAND3 cell 144 is cell “2”, etc.). Thisreferencing technique ensures that the path is reverse traversed, ratherthan forward traversed.

A determination is then made whether cell M is an LVT candidate 608.Recall that certain cells, such as transmission gates or latches, maynot be appropriate cells to implement LVT in. In FIG. 1, the first cellto be considered is the NAND3 cell 154. Because the NAND3 cell 154 isnot a cell whose functionality will change if some or all of its devicesare changed from SVT to LVT, and as long NAND3 cell 154 is not on anoise sensitive path, the NAND3 cell 154 is an appropriate candidate forLVT implementation.

If cell M is not an LVT candidate, M is decremented 612 and if it isthen not equal to zero 614 (i.e. there are still elements remaining onthat path), the flow returns to step 608; if M is equal to zero, flowcontinues to step 616. If cell M is an LVT candidate, and if the LVTswap improvement is positive (i.e. the swap results in a decrease inpropagation delay), then the required cycle time in the path report isupdated 610 for all of the paths in that design that contain cell M.This is done because implementing LVT in a cell will likely not onlyaffect the propagation delay for path N, but also will likely affect thepropagation delay for other paths. Also, if SVT devices in a cell otherthan those devices associated with a critical path, or the path beingreverse traversed in step 510 are replaced by LVT devices, other pathswill be affected. This may be the case, for example, where the standardcell library contains limited variations or no partial LVT cells. Ifthis is the case, other paths may need to be checked to determine theimpact of the SVT to LVT replacement on those other paths.

Continuing with the example of the circuit design in FIG. 1, if thechange in delay for the falling edge of the critical path 110 affectedif the NAND3 cell 154 is replaced with an LVT cell is a decrease of 3picoseconds, then the path report would be updated 610 by updating thecycle time of all of the paths in the design that contain cell 154,which in this example is only path 1.

Returning to step 609, if the LVT swap improvement is negative or doesnot exist, flow continues to step 612 to evaluate the next cell. M isdecremented 612, and if M is then not equal to zero 614 (i.e. there arestill elements remaining on that path), the flow returns to step 608. InFIG. 1, M would be decremented from 7 to 6, and the method wouldcontinue with step 608, considering cell “6” which is the NOR2 gate 152.

If M does equal zero (i.e. there are no more elements on the path), thenthat path reverse traversal is complete and flow continues to step 616.At step 616, N (the path number currently being traversed) isdecremented. At step 618, if N is not equal to zero, there are stillpaths in the design that need to be reverse traversed, and flowtransfers back to step 604. If N is equal to zero, there are no pathsleft to be reverse traversed, and the table is complete. For the circuitdesign in FIG. 1, there is only 1 path, and so following the calculationfor elements 154, 152, 150, 148, 146, 144, and 142 (in that order), Nwould be decremented to zero and the method illustrated in FIG. 6 wouldbe complete.

Once all of the paths in a design have been traversed and the pathreport has been updated, the path report is sorted and used toselectively implement LVT as illustrated in FIG. 5. For the FIG. 1example, if the reverse traversal and corresponding table of timingimprovements showed that replacing cells 142, 144, and 146 would resultin a timing improvement of 4 picoseconds each for both rising andfalling signals, replacing cells 148 and 152 would result in a timingimprovement of 2 picoseconds each for both rising and falling signals,and that replacing cells 150 and 154 would result in a timingimprovement of 5 picoseconds each for both rising and falling signals,the designer may wish to replace only cells 152 and 154 with acorresponding full or partial LVT cell. In this example, even thoughonly a few of the cells were replaced with cells containing some or allLVT devices, the replacements were efficient because they were the cellswhere the most timing improvement was to be gained by SVT to LVTreplacement.

It should be appreciated that the preceding method is an example only,and there are many steps that may be omitted, modified, or there mayalso be other steps introduced. For example, instead of reversetraversing each path in step 604, one aspect of this disclosure involvesforward traversal of the path. Reverse traversal, however, oftenprovides more efficient results because improving timing at the end of apath avoids the situation where earlier timing improvements getcancelled out by behavior of some intermediary element. Nonetheless,there may be situations where forward traversal provides more efficientresults.

Other design considerations may also need to be considered whenselectively replacing transistors, such as the need for footprintcompatible cells. The footprint of a cell refers to the surface areaused by the cell. Thus, an SVT to LVT footprint compatible cell is onewhere an LVT cell may replace with an SVT cell without violating designrules because it has a substantially similar footprint as an SVT cell.Note that there are some circuit and layout optimization techniques thatcan be used to realize an ‘effective’ size of a cell that do not affectthe actual footprint of the cell; these cells would still be consideredfootprint compatible. Modified cells may also need to pass other designrule checks, process specific requirements, and project specificrequirements.

Also, while aspects of this disclosure involve using LVT transistors inplace of SVT transistors, other aspects involve selectively using highthreshold voltage cells in combination with LVT and/or SVT transistors.High threshold voltage transistors, while slower than SVT and LVTtransistors, consume less power, and thus may be used in paths wheretiming is not critical in order to decrease power consumption.

As briefly mentioned above, there are often several side effectsassociated with LVT usage which may motivate a circuit designer to beselective in using LVT devices. Some of the many possible side effectsare increased power consumption, output conflicts, and minimum timingproblems caused by LVT devices. As a result, replacing all of thetransistors in a design with LVT devices is generally unacceptable. Evenimplementing LVT in all of the devices of an entire cell on a criticalpath may be disadvantageous.

One negative side effect that accompanies the increase in speed is thatLVT transistors consume more power than their SVT counterparts (incontrast, high threshold voltage devices leak less current but runfaster). For example, in 90 nm integrated circuit technology, LVTtransistors may leak as much as 40 times more current than an SVTdevice. The increase in power consumption is caused by both an increasedleakage current when the transistor is (supposed to be) off and by anincrease in the amount of time that the transistor is on. As transistorsand other electronic devices get smaller and smaller, the leakagecurrent tends to increase and the devices may be more prone to outputconflicts or timing violations.

While the increased power consumption caused by increased leakagecurrent negatively affects the amount of energy used (particularly forlogic circuits in mobile devices such as cell phones, laptops, and soforth), increased power consumption also negatively affects theoperating temperature of the circuit. Much of the power used in acircuit is dissipated through heat, which causes the temperature of thecircuit to increase. This temperature increase may negatively impactcircuit performance by, for example, slowing down transistors or othercircuit elements and preventing them from functioning correctly. Thus,although LVT is beneficial because of the decrease in propagation delay,the power and heat costs associated therewith may be one motivation tobe selective in implementing LVT at both the cell and device level.

Another drawback in changing all of the transistors in an individualcell to LVT transistors when only one pin (e.g. the pin on the criticalpath) is problematic may be that doing so may introduce minimum delaytiming problems on non-critical paths, thereby requiring a buffer to beinserted on the non-critical paths to actually increase the delay onthat path. This is illustrated in FIG. 1, where the signal 165 leavingclocked element 160 goes through a logic gate 154, which forillustrative purposes is a NAND3 gate. If all of the transistors in cell154 are replaced with LVT transistors, instead of only the transistorscorresponding to the pin on the critical path, signal 165 may propagatequickly through logic gate 154, thereby causing a minimum timeviolation, perhaps by violating the setup or hold times for clockedelement 130. Therefore, only the transistors corresponding to the pin ofgate 154 that is on the critical path may be replaced with LVTtransistors, leaving all of the other transistors as SVT devices, inorder to avoid introducing minimum time problems.

Yet another side effect that may encourage selective rather thanwholesale implementation of LVT is that LVT transistors may causeproblems by introducing output conflicts which not only propagate tosubsequent cells but can also drastically increase power consumption. Anexample of an output conflict is a situation where a pull up transistor(usually p-type) and a pull down transistor (usually n-type) coupledtogether in a cell are both on, establishing a conducting path from thepower supply to ground and leaving the output voltage unstable. Theindeterminate output voltage may cause the transistor and subsequenttransistors to enter a metastable state.

Such output conflicts often result when SVT cells are converted to LVTcells in the conventional manner, specifically by using SVT cells thathave been created and optimized for SVT devices and merely changing someor all of the SVT devices to LVT devices. This change, unaccompanied byany other modifications, may introduce unwanted side effects, such asthe output conflicts described above. These side effects may bemitigated by redesigning the cell or the individual devices within thecell in order to optimize performance and minimize the negative effectsintroduced by the SVT to LVT change.

Redesign may include, but is not limited to, resizing the transistorsand rearranging the circuit topology. For example, consider the NAND3gate 710 with some SVT and some LVT devices in FIG. 7A. In particular,transistors 732 and 738 are LVT devices, while transistors 730, 734,736, and 740 are SVT devices. This gate may have been created using astandard SVT NAND3 gate and simply changing some of the SVT devices toLVT devices. A problem arises if the LVT p-type transistor 732 has alonger than necessary conflict with the corresponding 738 n-typetransistor, or either of the other two n-type transistors 736 or 740,which prevents the output 750 from settling until the conflict isresolved.

The indeterminate output problem of the NAND3 cell in FIG. 7A might besolved by doubling the size of the p-type transistor 742 that waschanged to LVT and at the same time changing the topology of the n-typetransistors 738 and 736 as shown in FIG. 7B.

Similarly, SVT to LVT replacement often times results in a cell whoseoutput is skewed. In other words, using LVT in a cell may only improvethe propagation delay for either the rising or the falling output, withthe output for the other clock edge is actually degraded. This problemis exacerbated when the input slew rate is worse than normal, as isoften the case in critical paths of digital logic circuits.

If a cell has a degraded rising output after partial or full LVTreplacement, the size of n type transistor associated with the pin forthat output may need to be reduced (or the size of the p type transistorassociated with that pin may need to be increased). If the fallingoutput is degraded, after partial or full LVT replacement, then the sizeof the p type transistor associated with the pin for that output mayneed to be reduced (or the size of the n type transistor associated withthat pin may need to be increased if space allows). Determining how muchredesigning to do may, however, depend on how bad the expected inputskew rate is.

Note that there are several phases of circuit design where cell redesignmay be appropriate. Some aspects of this disclosure involve redesigningfull LVT and partial LVT cells by individual circuit designers duringdesign. Other aspects involve cell redesign at the library level, beforeindividual circuit design. If the cells are redesigned at the librarylevel, individual designers are spared the effort associated withredesign, which may be significant. However, even cell redesign at thelibrary level is likely to involve many hours of human effort and maynot be practicable. Thus, other aspects of this disclosure involvepartial redesign at the library level and continuing the redesignprocess in individual circuit designs.

As mentioned above, in step 508 in FIG. 5, actually replaced SVT deviceswith LVT devices depends greatly on the availability of full LVT andpartial LVT cells in the standard cell library. Accordingly, someaspects of this disclosure relate to a generalized standard cell librarywith full SVT, partial LVT, and full LVT cells (full SVT and full LVTcells may alternatively be called non-mixed threshold voltage cells, andpartial LVT cells may alternatively be called mixed threshold voltagecells) all of which have been designed or redesigned to optimizeperformance. In theory, it is possible to construct a comprehensivelibrary with all of the different combinations of SVT and LVT deviceswithin each and every different type of cell. However, the overheadassociated with creating and maintaining such a vast library is likelyprohibitive. This is demonstrated by considering all of thepossibilities that a standard NAND3 gate may entail: all 6 transistorsbeing SVT, 5 transistors being SVT and 1 being LVT, 4 transistors beingSVT and 2 being LVT, etc. Furthermore, there are multiple permutationsof these cells, i.e. the first 5 transistors being SVT and the lastbeing LVT, the first 4 and last 1 transistors being SVT and the secondto last transistor being LVT, etc. And the NAND3 gate is only one ofthousands of different logic gates. Furthermore, in order to optimizeperformance, each different version should be redesigned after replacingsome or all SVT devices with LVT devices.

There is thus a need to balance the library overhead with the neededspeed improvement and power savings. Some instances, such as where thereis an extreme need to optimize speed and power consumption, may justifyconsiderable effort in constructing a comprehensive library. The extraexpense of library construction may also be justified in the case of themost often used cells, such as the standard buffer and inverter. Inother situations, the redesign of the hundreds or thousands of differentstandard library elements may be left up to individual designers on anas needed basis.

While all different combinations and permutations of SVT and LVT deviceswithin cells is within the scope of this disclosure, one implementationsimplifies the complexity associated with a comprehensive library byonly having a few of the SVT/LVT combinations.

One implementation of a simplified standard library includes one versionof each cell with all SVT devices, one version with all LVT devices, andone version with only the transistors associated with one pin being LVTdevices (i.e. the n and p type transistors associated with that onepin). Using this implementation, the choice in step 508 may be to usethe one-pin LVT cell if only one pin of a cell in question is on acritical path, use the full LVT cell if at least two pins of the cellare on critical paths, or use the full SVT cell if no pins of that cellare on the critical path.

Another implementation of the simplified standard library includes oneversion of each cell with only one SVT p type transistor replaced withan LVT p type transistor, and one version with only one SVT n typetransistor replaced with an LVT n type transistor. Other implementationsmay include any other combination of SVT and LVT devices within thecells. Any of these limited standard libraries are still extremelyflexible and adapted to the needs of high speed, low power digital logiccircuit design when used in connection with pin swapping techniques.

Pin swapping techniques allow for the inputs to a cell to be switchedafter place and route without having to rearrange the contents of thecell. However, because of the effort that is likely associated with pinswaps after place and route (such as making sure the pin swap doesn'tchange any subsequent functionality or introduce new timing problems),some aspects of this disclosure involve carefully implementing partialLVT cells during the design phase and before place and route. Suchcareful implementation may include, but is not limited to, usingcharacterization information for the partial LVT cell contained in thecell library to determine which pin is the fastest pin in the cell, andusing the transistors associated with that pin for signals on what willmost likely be a critical path. Other aspects of this disclosure,however, involve pin swapping after place and route, and after timinganalysis has been performed.

The invention claimed is:
 1. A method for selectively replacing standardthreshold voltage devices with low threshold voltage devices in adigital logic design, comprising the operations of: identifying at leastone path in a digital logic design, the at least one path having aplurality of standard threshold voltage devices, the at least one pathhaving a first timing value determined by a timing engine; reversetraversing the path to identify at least one of the plurality ofstandard threshold voltage devices to replace with a corresponding lowthreshold voltage device, wherein the low threshold voltage device has alower threshold voltage needed to activate the device than the standardthreshold voltage device; determining, utilizing the timing engine, atleast one second timing value for the path associated with replacing theat least one standard threshold voltage device with the correspondinglow threshold voltage device; and replacing the at least one standardthreshold voltage device with the corresponding low threshold voltagedevice when the second timing value is less than the first timing value.2. The method of claim 1 wherein the at least one path is a criticalpath in the digital logic design.
 3. The method of claim 2 wherein thecritical path has the first timing value including a propagation delayof the plurality of standard threshold voltage devices of the criticalpath and the digital logic design has a maximum allowable propagationdelay; and wherein the operation of replacing reduces the propagationtime on the at least one critical path below the maximum allowablepropagation delay.
 4. The method of claim 1 wherein the operation ofreverse traversing the path further comprises: determining that the atleast one of the plurality of standard threshold voltage devices is notsusceptible of replacement.
 5. The method of claim 1 wherein the firsttiming value is a path cycle time including a propagation delay of eachof the plurality of standard threshold voltage devices of the at leastone path, a setup and hold time of a destination flop in the at leastone path, and a clock skew between a source flop and a destination flopof the at least one path.
 6. The method of claim 1 further comprisingthe operation of: identifying first standard threshold voltage device ata beginning of the at least one path and a second standard thresholdvoltage device at the end of the at least one path; and wherein theoperation of reverse traversing the at least one path begins with thesecond standard threshold voltage device and ends with the firstthreshold voltage device in order to prioritize the second standardthreshold voltage device over other standard threshold voltage devicesalong the at least one path for replacement.
 7. The method of claim 6wherein the beginning of the at least one path includes a non-clockedelement immediately following a first clocked element, and the end ofthe at least one path includes an element immediately preceding a secondclocked element.
 8. The method of claim 1, further comprising: whereinthe first timing value corresponds with a propagation delay introducedby the plurality of standard threshold voltage devices; wherein thesecond timing value corresponds with a second propagation delayintroduced by replacing the at least one standard threshold voltagedevice with the corresponding low threshold voltage device; generating atable including an indication of the at least one path and a timingdifference between the first and the second timing values along withtiming differences for other paths sorting the table based on the timingdifferences; and replacing only those standard threshold voltage deviceswhere replacement with the corresponding low threshold voltage devicewill result in the timing value reduction meeting a predetermined value.9. The method of claim 1 wherein the standard threshold voltage devicefurther comprises a plurality of p-type devices; and wherein a pluralityof p-type devices in the corresponding low threshold voltage device arereplaced by low threshold voltage p-type devices in order to reduce therising time of the corresponding element.
 10. The method of claim 1wherein: the standard threshold voltage device further comprises aplurality of n-type devices; and wherein a plurality of n-type devicesin the corresponding low threshold voltage device are replaced by lowthreshold voltage n-type devices in order to reduce the falling time ofthe corresponding element.
 11. The method of claim 1 wherein thestandard threshold voltage device further comprises at least one n-typedevice and at least one p-type device, the at least one n-type deviceand at least one p-type device corresponding to at least one input; andwherein only the n-type and p-type standard threshold voltage devicescorresponding to the at least one input are replaced by low thresholdvoltage devices in order to reduce the rising and falling timeassociated with the at least one input.
 12. The method of claim 1wherein the standard threshold voltage device further comprises aplurality of devices; and wherein all of the devices in thecorresponding low threshold voltage device are replaced by low thresholdvoltage devices.
 13. A system for selectively replacing standardthreshold voltage devices with low threshold voltage devices in adigital logic design comprising: a timing engine configure to obtain afirst timing value for at least one path in a digital logic design, theat least one path having a plurality of standard threshold voltagedevices; the timing engine further configured to reverse traverse thepath to identify at least one of the plurality of standard thresholdvoltage devices to replace with a corresponding low threshold voltagedevice provided in a database of low threshold voltage devices, whereinthe low threshold voltage device has a lower threshold voltage needed toactivate the device than the standard threshold voltage device; thetiming engine further configured to determine at least one second timingvalue for the path associated with replacing the at least one standardthreshold voltage device with the corresponding low threshold voltagedevice in order to replace the at least one element with thecorresponding element when the second timing value is less than thefirst timing value.
 14. The system of claim 13, the timing enginefurther configured to identify a first standard threshold voltage deviceat a beginning of the at least one path and a second standard thresholdvoltage device at the end of the at least one path; and wherein theoperation of reverse traversing the at least one path begins with thesecond standard threshold voltage device and ends with the firstthreshold voltage device in order to prioritize the second standardthreshold voltage device over other standard threshold voltage devicesalong the at least one path for replacement.
 15. The system of claim 14wherein the beginning of the at least one path includes a non-clockedelement immediately following a first clocked element, and the end ofthe at least one path includes an element immediately preceding a secondclocked element.
 16. The system of claim 13 wherein: the first timingvalue corresponds with a propagation delay introduced by the pluralityof standard threshold voltage devices; the second timing valuecorresponds with a second propagation delay introduced by replacing theat least one standard threshold voltage device with the correspondinglow threshold voltage device; the timing engine configure to: generate atable including an indication of the at least one path and a timingdifference between the first and the second timing values along withtiming differences for other paths; sort the table based on the timingdifferences; and replace only those standard threshold voltage deviceswhere replacement with the corresponding low threshold voltage devicewill result in the timing value reduction meeting a predetermined value.17. The system of claim 13 wherein: the standard threshold voltagedevice further comprises a plurality of p-type devices; and a pluralityof p-type devices in the corresponding low threshold voltage device arereplaced by low threshold voltage p-type devices in order to reduce therising time of the corresponding element.
 18. The system of claim 13wherein: the standard threshold voltage device further comprises aplurality of n-type devices; and a plurality of n-type devices in thecorresponding low threshold voltage device are replaced by low thresholdvoltage n-type devices in order to reduce the falling time of thecorresponding element.
 19. The system of claim 13 wherein the standardthreshold voltage device further comprises: at least one n-type deviceand at least one p-type device, the at least one n-type device and atleast one p-type device corresponding to at least one input; and whereinonly the n-type and p-type standard threshold voltage devicescorresponding to the at least one input are replaced by low thresholdvoltage devices in order to reduce the rising and falling timeassociated with the at least one input.
 20. The system of claim 13wherein: the standard threshold voltage device further comprises aplurality of devices; and all of the devices in the corresponding lowthreshold voltage device are replaced by low threshold voltage devices.